Effect of solvent on formation characteristics of sol-gel aluminium titanate

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Nitrogen-containing aluminium titanate

In the Ti-Al-O-N system a phase isostructural to aluminium titanate but with expanded unit cell dimensions was observed. It was stable between 1400 and 1700 °C and has unit cell dimensions of a=0.3719 nm, b=0.9703 nm and c=0.9869 nm with a composition of Ti _1.00 ⁴⁺ Al _0.54 ³⁺ Ti _1.46 ³⁺ N _0.28 ^3– O _4.58 ^2– _0.14 Several samples were prepared by reaction sintering mixtures of TiN, Al₂O₃ and AlN powders at 1400 to 1470 °C for 4 h in a nitrogen atmosphere to maximize this phase. One specific advantage of the nitrogen-containing aluminium titanate over aluminium titanate is that the former is unchanged at 1150 °C in a nitrogen atmosphere whereas the latter decomposes. In the Al₂O₃-TiO₂ oxide system Al₂TiO₅ solid solution extends to approximately Al_0.75Ti_2.25O₅ at 1470 °C under the mildly reducing conditions of a graphite furnace. The unit cell volume increases linearly with the increasing replacement of Al³⁺ by Ti³⁺.     

Thermal stabilization of aluminium titanate and properties of aluminium titanate solid solutions

Aluminium titanate has a near zero thermal expansion coefficient (=0.8×10^–6 °C^–1) in the range 20 to 1000 °C, nevertheless it decomposes below 1200 °C.The thermal stabilization of Al₂TiO₅ without altering its thermal expansion has been considered by partial substitution in the structure compound of Al³⁺ ions by Fe³⁺ ions.The solid solutions prepared by solid state reaction are in agreement with the general formula Al(1–x)₂Fe_2x TiO₅(0<x<0.2)The iron ions present in the crystal structure of Al₂TiO₅ act on its lattice parameters and bring about a catalytic effect in the formation of materials.Solid solutions show a strong thermal stability and a thermal expansion coefficient specially for the solid solution (x=0.1) which is not far from the Al₂TiO₅ value even after annealing for 300 h at 1000 °C.The mechanical properties of such materials corresponding to that solid solution present strength values lower than Al₂TiO₅ ones. After annealing, however, these are improved later due to a microcrystallization.     

Effect of the powder precursor characteristics in the reaction sintering of aluminum titanate

Aluminum titanate precursor powders, containing 2.5 wt.% MgO additive, were prepared by two powder processing methods, the hydrolysis of alkoxides and coprecipitation of inorganic salts. The hydrolysis-derived precursor powder showed a higher chemical homogeneity than the coprecipitated-derived precursor powder. Dilatometric analysis, accomplished with XRD and microscopy, was a very good method to study both phase transformation and sintering processes. Different phase transformation paths of TiO₂ and Al₂O₃ were observed in the two powders. The formation of aluminum titanate solid solution began at ∼1220°C in both, but in the hydrolysis-derived powder compact the aluminum titanate grains grew faster, leading to incomplete reaction and a non-uniform microstructure of large grains with small residual inclusions.     

A study of the stabilization of aluminium titanate

A tialite ceramics (Al₂TiO₅) was synthesized at a temperature of 1500 °C, incorporating CaF₂, La₂O₃, SiO₂ or kaolin and MgO additives. Its thermal stability was investigated by thermocycling in a reducing medium. The batches containing SiO₂ or kaolin additives underwent a decomposition to rutile and corundum. A stabilized ceramic with added MgO was produced. X-ray and electron-probe microanalysis established the presence of Mg/Al, Ti_z/O₄, Al_2–x-y Ti_1+x Mg _y O₅ and Ca_1–x La _x /Al_12–y-z Mg _y Ti _z /O₁₉ solid solutions, which retained their chemical composition after thermocycling in a reducing medium.     

Thermal expansion of grain-boundary cracked aluminium titanate ceramics

The relation between thermal expansion coefficient and grain-boundary crack volume of aluminium titanate ceramics has been calculated based on the thermal expansion data of a single crystal. The calculation began with the drawing of a frequency distribution curve of the thermal expansion coefficient for a single grain. Using the single-grain distribution curve, a distribution curve of an average coefficient for two adjacent grains was derived. The thermal expansion with a partially grain-boundary cracked body was calculated in relation to the amount of cracked boundary from the average distribution curves. The expected relation between crack volume and thermal expansion was close to that of the experimental data.     

Reformation of phase composition in decomposed aluminium titanate

The ability of decomposed Al₂TiO₅ to undergo reformation in phase composition has been characterised by neutron diffraction at elevated temperature and verified by differential thermal analysis. It is shown that the process of phase decomposition in metastable Al₂TiO₅ is reversible and that reformation occurs readily when decomposed Al₂TiO₅ is re-heated above 1300 °C. Subsequent decomposition of reformed Al₂TiO₅ during cooling below 1200 °C is governed by the temperature-dependent atomic diffusion rates.     

Effect of zirconia additions on the reaction sintering of aluminium titanate

Yttria-stabilized zirconia was added to a chemically prepared mixture of alumina and titania. The effect of the zirconia on the microstructure and resultant properties was studied following reaction sintering to form aluminium titanate. An increase in mechanical strength was observed with little effect on the excellent thermal properties of the aluminium titanate. This was attributed to generation of extra microcracks by the transformation of the zirconia phase and the unusual microstructure produced by the presence of zirconia.     

Dependence of the refractive index grating in photorefractive barium titanate on intensity

The interference pattern of light waves creates a refractive index modulation in photorefractive media. This process is relatively well described by the theory (Kukhtarev equations) but deviations are found. Therefore, experimental methods are used in order to characterise the processes. The influence of the absolute intensity as well as the intensity ratio of the interfering waves on the refractive index modulation are studied in a two-wave mixing arrangement. Especially for the dependence on the absolute intensity the interesting relation Δn=f(I_abs)I_abs was found that is not predicted by the theory. Nevertheless, this experimental approximation can be used in calculations in order to minimise the error.     

Mechanochemical effects on the kinetics of zinc titanate formation

In this work, mixtures Zn-TiO₂ (anatase) in molar ratio 1:1 were mechanochemically activated in air atmosphere, and submitted to thermal treatments in order to study its thermal transformations. The behavior of the system during the milling was followed by X-ray diffraction (XRD), differential thermal analyses (DTA) and thermogravimetric analyses (TGA). Mechanochemical activation produces a progressive loss in crystallinity of the starting powders, with simultaneous oxidation of metallic Zn. However, the formation of neither ZnTiO₃ nor Zn₂TiO₄ could be detected. At temperatures above 600°C, the thermal treatments resulted in the formation of ZnTiO₃ and Zn₂TiO₄, at lower temperatures and shorter holding times for samples activated during longer times. The non-activated mixture exhibited a very different behavior, yielding Zn₂Ti₃O₈ and Zn₂TiO₄ without evidence of ZnTiO₃ formation. The obtained results are explained on the basis of reaction mechanisms taking place in the activated and non-activated samples.     

Dissolution/recrystallization growth of titanate nanostructures by amorphous precursor

Understanding of growth mechanism is of technical importance for tailoring the size and morphology of titanate nanostructures. However, the growth mechanism of titanate nanostructures in alkali solution systems by using crystalline TiO₂ remains debating currently. In the present work, the amorphous precursor of titanium hydroxide precipitates, a highly disordered raw material, was used as the precursor to prepare the titanate nanostructures under hydrothermal conditions. SEM and TEM results show that the morphology of the titanate nanostructures developed from nanoparticles to nanosheets and then the titanate nanowires with an interlayer spacing of 0.786 nm as the reaction time prolonged. XRD and Raman spectra results display that layered titanate nanostructure were formed. These phenomena are similar to that of the titanate nanostructures prepared by the TiO₂ crystal in alkali solution systems. The findings provide direct evidence to strongly support that titanate nanostructures grow from dissolution/recrystallization process under hydrothermal process, allowing mediating the size and morphology of titanate nanostructures. Moreover, SEM and photocatalytic results implied that the washing process improved the photocatalytic activities, which had no effect on the overall morphology.     

Thermal cycling of iron- and magnesium-containing aluminium titanate in a dilatometer

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

The effects of additives on the properties and structure of hot-pressed aluminium titanate ceramics

The effects of additives (MgO-Al₂O₃-SiO₂, Al₂O₃-SiO₂, Si₃N₄, SiC) on the properties and structure of hot-pressed aluminium titanate ceramics were studied. The results showed that the bending strength of aluminium titanate ceramics with additives was improved greatly, while the thermal expansion coefficient was also increased due to the change of structure for aluminium titanate ceramics. The results of X-ray diffraction patterns showed that a solid state reaction occurred between the additives and aluminium titanate, and a solid solution was formed.     

Low temperature synthesis of aluminium titanate by an aqueous sol-gel route

Formation of aluminium titanate (AT) has been achieved at low temperature through sol-gel process using boehmite and titanium hydroxide as precursors by controlling the particle size at nanoscale followed by in-situ peptisation. The formations of AT phase, particle size distributions, sintering and thermal expansion characteristics, and microstructural features have been reported. DTA and XRD analysis have been performed to confirm the formation of AT. A 94% relative density was obtained for aluminium titanate sintered at 1550 °C with controlled grain size in the range of 2-3 μm.     

Phase formation and characteristics of r.f.-sputtered barium-strontium titanate thin films on various bottom layers

(Ba_0.7Sr_0.3)TiO₃ thin films were deposited by r.f.-magnetron sputtering on Pt/Ti/Si, Pt/TiSi₂/Si and Pt/Ti/SiO₂/Si substrates, respectively, and annealed at 650 C for 30s by Rapid Thermal Annealing (RTA). XRD (X-ray diffraction) patterns revealed that the BST films had perovskite structure without preferred orientation. Auger depth profiles of barium-strontium titanate (BST) films on various substrates were performed. In the Pt/Ti/Si and Pt/TiSi₂/Si structures, Si diffused into the BST film, but in the Pt/Ti/SiO₂/Si structure, the diffusion of Si into the BST film was prevented and the interface between the BST film and the electrode was stable. The dielectric constants were about 310–260 (100 kHz–1 MHz).     

Nanosilver particle formation on a high surface area titanate

Titanium based molecular sieves, such as ETS-10, have the ability to exchange silver ions and subsequently support self assembly of stable silver nanoparticles when heated. We report that a high surface area sodium titanate (resembling ETS-2) displays a similar ability to self template silver nanoparticles on its surface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show high concentrations of silver nanoparticles on the surface of this sodium titanate, formed by thermal reduction of exchanged silver cations. The nanoparticles range in size from 4 to 12 nm, centered at around 6 nm. In addition to SEM and TEM, XRD and surface area analysis were used to characterize the material. The results indicate that this sodium titanate has a high surface area (>263 m2/g), and high ion exchange capacity for silver (30+ wt%) making it an excellent substrate for the exchange and generation of uniform, high-density silver nanoparticles.     

Study on the formation and growth of potassium titanate whiskers

In this paper, K₂Ti₂O₅ single crystals, K₂Ti₄O₉ whiskers and K₂Ti₆O₁₃ whiskers are synthesized form the anatase-K₂CO₃ starting materials by the heating calcination and the corresponding morphologic and structural evolution of products are observed. After dissolving non-crystalline hydrosoluble products contained in sinters, the morphologic difference between the original sinter and the whiskers in it shows the sinter microstructure. The further analysis to crystal components in sinters and to the phase diagram proves that K₂Ti₄O₉ whiskers are firstly formed in K₂Ti₂O₅ crystals and there only exists the phase transformation from K₂Ti₄O₉ whiskers with layered crystal structure to~K₂Ti₆O₁₃ whiskers with tunnel crystal structure. The K₂O-rich liquid melt generated from K₂Ti₂O₅ crystals (/and K₂Ti₄O₉ whiskers) coats on the surface of K₂Ti₄O₉ whiskers (/and K₂Ti₆O₁₃ whiskers), which makes the sinters taking on the layer-by-layer structure (/and the bunch structure). The formation and growth of whiskers is dictated by the K₂O-rich non-crystalline hydrosoluble melt generated in phase transformations from solid to liquid-solid and its split effect induced by the orientation melting. A generalized liquid melt inducing mechanistic model explaining the formation and growth of potassium titanate whiskers was proposed.